In order to prevent reflection at the light incident/emitting surface of an optical member, it has been known to form an antireflection film of a monolayer or multilayer of optical films having different refractive indices laminated in a thickness of several tens to several hundreds nanometers. Such an antireflection film is formed by a vacuum deposition process such as vapor deposition or sputtering or a wet film-formation process such as dip-coating or spin-coating.
As the material for the outermost layer of the antireflection film, transparent materials having low refractive indices, for example, inorganic materials such as silica, magnesium fluoride, and calcium fluoride and organic materials such as silicone polymers and amorphous fluorine polymers are known.
In order to further reduce reflectivity, it is recently known to use a low-refractive-index film, which utilizes the fact that air has a refractive index of 1.0, in the antireflection film. Refractive index can be reduced by forming voids in a layer of silica or magnesium fluoride. For example, the refractive index of a magnesium fluoride thin film can be reduced from 1.38 to 1.27 by providing 30 vol % of voids in the film.
An antireflection film having a low refractive index can be prepared by forming voids between fine particles of silica or magnesium fluoride through film formation using the fine particles together with a binder (PTL 1 and PTL 2).
In another method of forming voids, the voids are formed using hollow silica particles. In this method, an antireflection film is formed using these hollow particles (PTL 3).
It is known, however, that the use of hollow silica particles in optical members such as the antireflection film causes problems in transparency and appearance. This is due to that the hollow silica particles have low affinity to the organic solvent or the organic polymer that is mixed with the hollow silica particles and aggregate during preparing a coating material to cause scattering. In order to solve this problem, a film is formed by a coating material not containing an organic polymer to prevent aggregation, and an antireflection film is formed using this low-refractive-index film not causing scattering (PTL 4).
In order to prevent scattering, the antireflection film described in PTL 4 does not contain a binder represented by organic polymers. A film not containing a binder has a problem of insufficient abrasion resistance, since the outermost layer of an antireflection film is required to have abrasion resistance as well as a low refractive index and transparency.
In the case of using voids between particles and also forming voids inside a binder in order to further reduce the refractive index, voids become ununiform by, for example, aggregation of particles and void sizes locally increase to cause a problem of insufficient transparency.